Various anticorrosion coating compositions for metals, especially iron and steel, are known. One of the most effective is a composition comprising chromic anhydride or chromic anhydride and a boric acid compound, powder of a base metal (usually zinc or aluminum), a pH regulator (oxide or hydroxide of a metal), a low-molecular weight oxohydroxyether or glycol and water and/or an organic solvent.
This composition is usually supplied to users in the form of a combination of a first composition comprising chromic anhydride and optionally boric acid compound, a pH regulator and water and/or water-miscible organic solvent and a second composition comprising a metal powder and low-molecular weight oxo-hydroxyether or glycol (propylene glycol, for instance). The two are mixed together prior to use and the mixture is applied to the surface of metals to a predetermined thickness (to an extent that a coating layer having a thickness of 1 micron or more is finally formed) and subjected to the heat treatment at not lower than 200.degree. C. for at least 0.2 sec.
This composition exhibits an excellent effect for preventing corrosion of iron and steel against salt water, which is superior to zinc plating. Also this composition exhibits excellent performance over a long period of time in the continuous salt spray test. However, even this composition does not always perform satisfactorily in stricter accelerated tests such as the complex cycle test, which is often employed recently. Said complex accelerated test comprises a salt spray test, a moistening and drying test, etc. conducted in combination. That is, the above composition is not sufficient in very severe environments, where superior corrosion prevention performance is expected.
Although the mechanism of corrosion prevention of this composition is not entirely understood, it is surmised that chromium oxides (reduction products of chromic acid, mainly Cr.sub.2 O.sub.3) act as a binder (matrix) for the metal powder to form a coating layer on the surface of the substrate metal and passivate and stabilize the metal surface. Also the matrix has slight electric conductivity and hexavalent chromium remaining in the chromium oxides maintains the metal powder active and thus they causes the metal powder to exhibit sacrificial protective activity. It is known that the protective effect of this composition in salt water environments can be enhanced by increasing the content of the chromium oxides in the coating layer. However, if a larger amount of hexavalent chromium is used, this is a problem from the viewpoint of environmental pollution. That is, in the courses of production and disposal, it is required to convert the hexavalent chromium to harmless substances by reduction and this leads to increased manufacturing and disposal costs.
In order to create an improved anticorrosion coating composition having superior anticorrosion protective performance under severe salt corrosion environments without using a larger amount of chromium compounds, we prepared a number of compositions of various metal compounds and conducted tests on corrosion prevention performance with respect to the prepared compositions. Out of the tested metallic compounds, we found compounds of nickel (Ni) and/or cobalt (Co) to be effective and created an anticorrosion coating composition by addition of compounds of these metals. Compared to the known composition, the composition is less harmful to the environment, has a dramatically improved anticorrosion performance in salt water environments and exhibits stabilized corrosion prevention effect in freshwater environments.